Power supply protective control

ABSTRACT

An improved protective circuit provided by which a silicon controlled rectifier (SCR), or equivalent device, is switched into circuit with a power supply in the presence of excessive output voltage, so as to protect the electronic equipment which is normally energized by the power supply. The protective circuit of the invention is such that the silicon controlled rectifier is not subjected to excessive currents so that it need not be excessively large, and no heat sink is required in conjunction with the silicon controlled rectifier.

United States Patent [151 3,660,724 Berger [4 1 May 2, 1972 s41 POWERSUPPLY PROTECTIVE 3,217,207 11/1965 Webb ..317/51 T L 3,408,553 10/1968Bishop ....32l/l8 X 3,427,528 2/1969 Custer ..32l/18 [72] Inventor:James K. Berger, Sherman Oaks, Calif. [73] Assignee: giglriiteerMagnetics, Inc., Santa Monica, Zigfig ifgggggfgg' Tramme" [22] Filed:Mar. 18, 1971 [5 ABSTRACT 21 APPL 125 54 An improved protective circuitprovided by which a silicon controlled rectifier (SCR), or equivalentdevice, is switched into circuit with a power supply in the presence ofexcessive [52] U-S.Cl...., ..3l7/l6,3l7/31,321/2, output voltage, so asto protect the electronic equipment 3 1 l which is normally energized bythe power supply. The protec- [51] Int. Cl. ..H02h 3/20 tive circuit ofthe invention is such that the silicon controlled [58] Field of Search..3l7/16, 31, 51; 321/2, 18, rectifier is not subjected to excessivecurrents so that it need 321/1 1, 14 not be excessively large, and noheat sink is required in conjunction with the silicon controlledrectifier. 56 i l 1 Memes C M 3 Claims, 1 Drawing Figure UNITED STATESPATENTS 3,366,866 1/1968 King ..321/14 (11L I100 (0 3r T 01%? l 2/ g 1HC37 12 my Ka fflfl/ffl/ g C/kc'l/i/ A (23 ,eg 7! POWER SUPPLY PROTECTIVECONTROL BACKGROUND OF THE INVENTION Copending application Ser. No.58,042, filed July 270, in the name of the present inventor, isconcerned with an improved type of power supply which has particularutility for energizing electronic equipment. The power supply describedin the copending application is a regulated fly-back type in whichelectrical energy from an appropriate source is alternately stored in anelectromagnetic device, such as a transformer, and then released into aload.

In power supplies of the type described in the copending application,for example, it is usual to connect a silicon controlled rectifier (SCR)directly across the output of the power supply, and the SCR is firedshould the output voltage become excessively high to protect theelectronic equipment which is energized by the power supply. The siliconcontrolled rectifier (SCR) provided in the prior art protective circuitsmust be large since it must handle extremely high currents, and it mustbe mounted on a large heat sink. This is because the protective SCR mustbe capable of withstanding the largest fault current that might occur.

As noted above, the objective of the present invention is to provide ameans for connecting the silicon controlled rectifier (SCR), or anequivalent switching element, into the power supply circuit, so that itperforms the desired protective function, as described above, and yet ina way that it does not encounter high currents, so that a small SCR maybe used, and so that no heat sink is necessary. This is achieved inaccordance with the invention, as will be described by controlling thetransistor switch which connects the transformer to the input source bya separate transformer. Then, if a fault occurs, the SCR serves to blockthe input to the separate transformer so that the power supply cannotcontinue to operate until the fault has been corrected.

The circuit of the present invention, therefore, serves to move theprotective SCR from the high power output circuit, in which it isconnected in the prior art protective systems, back to a circuit pointwhich does not have high power, but which has the property in that nocomponent failure between that point and the output can produce anexcessive output voltage. No such point exists in the usual type ofseries-regulator power supplies, but it does exist in the type of powersupply described in the copending application, and in several otherswitching regulator type circuits. The present invention is predicated,therefore, on a principle in that the protective silicon controlledrectifier (SCR) is used to block the transformer input, rather than toabsorb directly the excessive transformer output load current, when afault occurs in the latter type of power supply circuit. It will beobserved that when the protective circuit of the invention is applied toa power supply of the type described in the aforesaid copendingapplication, no failure can occur which will allow energy to continue tobe transferred to the load, one the protective SCR has been fired.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE shows a circuitrepresentative of one embodiment of the invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT In the illustratedcircuit, a source of unidirectional potential designated V1 is connectedto one side of the primary winding type designated lN472 l. Thecollector of the transistor Q] is connected to the other side of theprimary of the transformer T1. the secondary of the transformer T1. isconnected through a diode CR2 to the output terminals'l0, 11 of thepower supply. A load filter capacitor C l is connected across theterminals 10. The diode CR2 may be of the type designated lNl 184R. Thecapacitor C1 may have a capacity of 48,000 microfarads. v

A control circuit 12 is connected to the primary winding of thetransformer T2 by leads l4 and 16. The lead 16 is connected to a furtherlead 18 through a diode CR3. The diode CR3 may be of the type designatedlN4l48. The lead 18 is connected to the output terminal 11. The outputterminal 10 is connected to a Zener diode CR4 which, in turn, isconnected through a resistor R1 to the gate electrode of a siliconcontrolled rectifier (SCR1). The Zener diode CR4 may be of the typedesignated 1N752. The resistor R1 may be have a resistance of 12 ohms.

The gate electrode of the silicon controlled rectifier (SCR1) isconnected through a resistor R2 to the lead 18, and the cathode of thesilicon controlled rectifier (SCR1) is directly connected to the lead18. The resistor R2 is shunted by a capacitor C2. The resistor R2 mayhave a resistance of ohms, and the capacitor C2 may have a capacity of4.7 microfarads. The gate electrode of the silicon controlled rectifier(SCR1) is connected to the lead 14 through a Zener diode CR5, and theanode of the silicon controlled rectifier (SCR1) is connected to thelead 14 through a diode CR6. The Zener diode CR5 may be of the typedesignated IN978, and the diode CR6 may be of the type designatedlN4l48. An additional diode, CR7, and a resistor R3 may be connectedwith the resistor between the anode of CR7 and the output terminal 10,and the cathode of the anode to the anode of the SCR.

When the output voltage falls below a pre-established level, the controlcircuit 12 applies a pulse to the primary winding of the transformer T2which causes the transistor O1 to become conductive. Current then flowsfrom the source V1 through the primary winding of the transformer T1,and through the transistor Q1 and diode CR1, and through the lower partof the secondary winding of the transformer T2, back to the source. Thiscurrent stores energy in the transformer T1, since the transformer T1has substantial inductive reactance.

When the stored energy is sufficient, the control circuit I2 essentiallyshort-circuits the primary winding of the transformer T2 which removesthe base current from the transistor 01, causing the transistor Q1 tobecome non-conductive. The energy in the transformer T1 is nowdischarged through the diode CR2 into the load filter capacitor cl, thusproducing the output voltage of the power supply. When the outputvoltage again decays to the present level, the cycle is repeated. Inthis way, the normal regulation of the output voltage of the powersupply is accomplished.

In the prior art, it was usual to connect a silicon controlled rectifierprotective device directly across the output terminals 10 and ll of thepower supply. Then, if the control circuit 12 should fail, and cause theoutput voltage to rise above the regulated level, the silicon controlledrectifier in the prior art system would be turned on, and it wouldabsorb all the output current of the supply, thus protecting theelectronic equipment normally energized by the power supply.

However, as pointed out, such a technique required that the siliconcontrolled rectifier SCR1) be of relatively large power handlingcapabilities, and heat sinks were also required inconjunction with thesilicon controlled rectifier (SCR1). However, in the circuit of thepresent invention, the silicon controlled rectifier (SCRI) is connectedto the control transformer T2 so that when the silicon controlledrectifier (SCR1) is fired, the primary winding of the transformer T2 isessentially short circuited through SCRI, and through the diodes CR6 andCR3. This short circuit is unidirectional, so that the power transistorQ1 cannot be rendered conductive, although the transformer T2 is free todevelop a voltage which further reverse biases the transistor Q1.

If SCRl is turned on, there is no possible failure other than of thediodes CR3 or CR6, or of the silicon controlled rectifier (SCRl) itself,which would allow energy to be transferred from the source to the load.If the transistor Q1 fails open, no energy can be stored in thetransformer T1. On the other hand, if the transistor Q1 fails shorted,the excessive primary current in the transformer T1 causes the shortcircuit to clear immediately by destruction of the interval connectionsin transistor Q1, thereby causing the transistor O1 to becomenon-conductive.

Therefore, the load is protected when the silicon controlled rectifier(SCRl) is turned on, just as surely as if the silicon controlledrectifier (SCRl) were connected directly across the output of the powersupply. However, the silicon controlled rectifier (SCRl) need handleonly a small control current, as compared with the heavy output currentwhich must be controlled by the protective silicon controlled rectifierin the prior art circuit. The diode CR6 allows the transformer windingto operate at high frequency during normal operation, without danger ofaccidental triggering of the silicon controlled rectifier SCRl due tothe rate of change of anode voltage. Diode CR7 is and resistor R3provide a current path to enable the SCR to discharge capacitor C1, whenthe SCR is fired. The resistor R3 serves to limit the discharge currentto a safe level.

In the circuit of FIG. 1, the diode CR4 is a Zener diode having abreakdown voltage slightly greater than the normal supply outputvoltage. If the supply output rises to the level of breakdown of theZener diode CR4, current flows from the output through the diode CR4 andthrough the current limiting resistor R1, into the gate of the siliconcontrolled rectifier (SCRl thereby firing the silicon controlledrectifier (SCRl The resistor r2 and capacitor C2 form a filter networkto prevent noise spikes from producing spurious firing of the siliconcontrolled rectifier (SCRl The illustrated circuit also serves toprotect the power transistor Q1 from excessive current due to controlfailure, and also serves to protect the control circuit 12 fromexcessive voltage due to failure of the power transistor Q1. the Zenerdiode CR5 performs both these functions. Excessive current in thetransistor Q1 will cause the base-emitter voltage of the transistor Q1,and the forward conduction voltage of the diode CR1 to become excessive.This will result in a voltage across the primary winding of thetransformer t2 which exceeds the breakdown voltage of the Zener diodeCR5. Current will then flow through the diode CR5 into the gate of thesilicon controlled rectifier SCRl, causing the silicon controlledrectifier to fire, thereby shutting oil the current in the powertransistor Q1.

Ifthe power transistor Q1 fails in a short circuiting condition, theprimary current through the transformer T1 becomes excessive, and wouldresult in a voltage on the primary winding of the transformer T2 whichwould damage the control circuit, except that before the voltage risesto dangerous level it exceeds the breakdown voltage of the Zener diodeCR5, and turns on the silicon controlled rectifier SCR]. This causes thevoltage to be clamped down to a few volts above the ground potential,and protects the control circuit from damage.

While a particular embodiment of the invention has been shown anddescribed, modifications may be made. It is intended in the followingclaims to cover all such modifications as fall within the spirit andscope of the invention.

I claim:

1. A power supply circuit including: a first transformer having aprimary winding and a secondary winding; a second transformer having aprimary winding and a secondary winding; a unidirectional potentialsource connected to one side of the primary winding of said firsttransfonner and to one side of the secondary winding of said secondtransformer; a switching transistor having a base electrode connected tothe other side of the secondary winding of said second transformer, anda collector electrode connected to the other side of the primary of saidfirst transformer, and an emitter electrode connected to a tap on thesecondary winding of said second transformer;

an output circuit connected to the secondary winding of said firsttransformer for developing an output voltage; a control circuitconnected to the primary winding of the second transformer forcyclically controlling the conductivity of said transistor to maintainsaid output voltage at a predetermined regulated level; and a protectivecircuit including a silicon controlled rectifier connected across theprimary winding of said second transfonner to short circuit saidlast-named primary winding whenever said silicon controlled rectifier isfired so as to prevent said transistor from being rendered conductive,and said silicon rectifier having a gate electrode connected to saidoutput circuit to cause said silicon controlled rectifier to firewhenever the output voltage exceeds a predetermined threshold.

2. The power supply circuit defined in claim 1, and which includes saidgate electrode to determine the voltage level at a Zener diodeinterposed between said output circuit and said gate electrode todetermine the voltage level at which said silicon controlled rectifierwill fire.

3. The power supply circuit defined in claim 1, and which includes aZener diode connected between the primary winding of said secondtransformer and the gate electrode of said silicon controlled rectifierto cause said silicon controlled rectifier to fire whenever the voltageacross the last-mentioned primary winding exceeds a predetermined value.

I! t l

1. A power supply circuit including: a first transformer having aprimary winding and a secondary winding; a second transformer having aprimary winding and a secondary winding; a unidirectional potentialsource connected to one side of the primary winding of said firsttransformer and to one side of the secondary winding of said secondtransformer; a switching transistor having a base electrode connected tothe other side of the secondary winding of said second transformer, anda collector electrode connected to the other side of the primary of saidfirst transformer, and an emitter electrode connected to a tap on thesecondary winding of said second transformer; an output circuitconnected to the secondary winding of said first transformer fordeveloping an output voltage; a control circuit connected to the primarywinding of the second transformer for cyclically controlling theconductivity of said transistor to maintain said output voltage at apredetermined regulated level; and a protective circuit including asilicon controlled rectifier connected across the primary winding ofsaid second transformer to short circuit said last-named primary windingwhenever said silicon controlled rectifier is fired so as to preventsaid transistor from being rendered conductive, and said siliconrectifier having a gate electrode connected to said output circuit tocause said silicon controlled rectifier to fire whenever the outputvoltage exceeds a predetermined threshold.
 2. The power supply circuitdefined in claim 1, and which includes said gate electrode to determinethe voltage level at a Zener diode interposed between said outputcircuit and said gate electrode to determine the voltage level at whichsaid silicon controlled rectifier will fire.
 3. The power supply circuitdefined in claim 1, and which includes a Zener diode connected betweenthe primary winding of said second transformer and the gate electrode ofsaid silicon controlled rectifier to cause said silicon controlledrectifier to fire whenever the voltage across the last-mentioned primarywinding exceeds a predetermined value.